Thermionic electrode with an auxiliary starting coil for a discharge lamp



July 7, 1970 P. c. WARD 3,519,372

THERMIONIC ELECTRODE WITH AN AUXILIARY STARTING COIL FOR A nzscmmem LAMPFiled May 17. 1967 FIG 4 INVENTOR Patrick C. Ward ATTORNEY United StatesPatent 3,519,872 THERMIONIC ELECTRODE WITH AN AUXILIARY STARTING COILFOR A DISCHARGE LAMP Patrick C. Ward, New York, N.Y., assignor toWestinghouse Electric Corporation, Pittsburgh, Pa., :1 corporation ofPennsylvania Filed May 17, 1967, Ser. No. 639,154 Int. Cl. H011 61/06,61/18, 61/54 US. Cl. 313-211 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION This invention generally relates tomercury-metal halide type arc-discharge devices, and particularly to animproved electrode design for such a lamp.

The so-called mercury-additive high-pressure discharge lamp has becomefamiliar in the commercial lighting field. Such a lamp is described inUS. Pat. N0. 3,234,421 dated Feb. 8, 1966. In one form, this type oflamp can use a resistive heating element as part of the structure toinitiate the discharge. Such an electrode is taught by Walter J. Deckerand Hugh D. Fraser in copending application Ser. No. 379,110 filed June30, 1964, now Pat. No. 3,356,884, entitled Arc-Discharge Device, andowned by the present assignee. The structure disclosed therein uses afine wire coiled tungsten heating element overwound about theextremities of two solid tungsten support rods. The fine wire coiltightly fitting about the solid support rod acts therewith as theprimary discharge sustaining surface. A coiled thermionic emitterassociated with the primary portion of the electrode to facilitateinitiating the discharge is taught by Hays et al., Pat. No. 2,263,171,dated Nov. 18, 1941, and also owned by the present assignee.

It is desirable when using such starting devices that the dischargeshift quickly, after it is initiated, from the heating portion of theelectrode to the more substantial primary discharge sustaining surfacefor improved lamp stability, and longer life of the starting element. Avery heavy coiled tungsten member which may or may not retain a low-workfunction material is known to provide such a substantial dischargesustaining surface. The disposition and composition of the electrodeelements determines this capability of quickly shifting the discharge,once initiated, from the starting element to the more substantialportion of the electrode.

SUMMARY It is an object of this invention to provide an improvedarc-discharge device which incorporates a very heavy coiled tungstenoperating electrode in conjunction with a fine wire heating element tofacilitate starting.

It is another object to provide a discharge lamp construction which willquickly stabilize the discharge on the primary discharge sustainingsurface after the heating element initiates the discharge.

The aforesaid objects and others which will become apparent as thedescription proceeds are achieved providing an arc-discharge devicewherein one electrode comprises two elongated tungsten support membersof unequal 3,519,872 Patented July 7, 1970 length. One end of eachsupport member is embedded in one seal of the arc tube. The longertungsten support member has a heavy coiled tungsten member tightlyoverfitting the inwardly extending extremity of the longer member,forming an operating discharge-sustaining surface. A material having alow-work-function may be retained by the heavy coiled tungsten memberand the elongated support member. A projecting arm portion is afiixed tothe inwardly extending extremity of this heavy coiled tungsten member.One end of a relatively fine wire coiled-coil tungsten heating member istightly fitted over this projecting arm portion. The other end of thecoiled-coil tungsten heating member is tightly fitted over the shorterelongated support member. The body of the coiled-coil tungsten heatingmember is thereby supported intermediate the shorter support member andthe projecting arm portion of the heavy coiled tungsten member which isthe primary discharge sustaining surface.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thisinvention, reference should be had to the accompanying drawings,wherein:

FIG. 1 is a sectional elevational view of a preferred lamp embodimentwhich incorporates the improved electrode of the prevent invention;

FIG. 2 is a view showing the electrode-heater element beforeincorporation into the arc tube;

FIG. 3 is a sectional elevational view of the arc-tube portion of thelamp as shown in FIG. 1, illustrating in detail the electrodes as sealedinto and positioned to the are tube;

FIG. 4 illustrates an alternative embodiment arc tube wherein bothelectrodes are constructed as shown in FIG. 2;

FIG. 5 is a view of an alternative embodiment of the electrode heaterelement before incorporation into the arc tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT With specific reference to theform of the invention illustrated in the drawings, the lamp 10 in FIG. 1generally comprises a radiation-transmitting, sealed inner quartzenvelope or are tube 12 having seals at either end thereof, withelectrodes 14 and 16 disposed proximate the arc tube seals and operableto sustain a vapor discharge therebetween. For operation with the powerinput of 400 watts, the arc tube 12 encloses a volume of 28 cc. Apredetermined charge of mercury 18 is included in the arc tube 12 in anamount of 117 milligrams and a predetermined amount of selected additivematerial such as 5 milligrams of thallous iodide 20, 26 milligrams ofsodium iodide 22, and 15 milligrams of thallium metal 23 are alsoincluded within the arc tube 12. Many other combinations of so-calledadditive metallic halides can also be used, as is known in the art. Asmall charge of inert ionizable starting gas, such as argon at apressure of 20-25 millimeters, completes the discharge-sustainingfillmg.

A radiation-transmitting, sealed, outer envelope 24 is spaced from andsurrounds the arc tube 12, and electrical lead-in conductors are sealedthrough both the inner envelope 12 and the outer envelope 24 and serveto electrically connect the operating electrodes 14 and 16 to a powersource. The volume between the outer envelope 24 and the arc tube 12preferably encloses a hard vacuum, although a nitrogen fill can be used.

The ends of the are tube 12 have provided thereabout metallic end capsor other suitable heat shields 26-. Aluminum silicate fiber or similarmaterial is desirably packed intermediate the end caps 26 and the aretube 12, in order to provide a good heat insulation effect and preventcondensation of the additive material at the ends of the arc tube. Thearc tube 12 is supported within the outer envelope 24 by a conventionalsupporting frame 28. Conventional ribbon seals 30 facilitate makingelectrical connection through the arc tube.

The electrode 16 is fabricated as an electrode-heating element and theheater element end thereof electrically connects through a switching orbimetal means 32, in order to facilitate starting of the device, asdescribed in detail in the aforementioned Pat. 3,307,069. One end of thebimetal 32 connects through the arc tube frame 28 and the base member 34to a conventional power source and the other end of the bimetal 32 ispermanently connected through an insulated flexible leadin conductor 36to the electrode 14.

The specified lamp is subject to considerable modification utilizingmaterials and techniques well known in the art. Lamps which are tooperate at greater or less input power will have design parameterseffecting such use, and the foregoing detailed specific example is notintended to limit these parameters.

In FIG. 2 is shown the electrode 16 prior to incorporation into the arctube. The elongated tungsten support members 40, 42 are also shown, Thelonger support member 40 has a relatively heavy coiled tungsten member44 tightly fitting about its extending extremity. Afixed to theextending extremity of the heavy coiled tungsten member 44 is aprojecting arm portion 46 generally directed toward but spaced from theshorter elongated member 42. One end 48 of the fine wire tungstencoiled-coil heating element 50 is tightly overfitted on this projectingarm portion 46. The other end 49 of the coiled-coil heating element 50is tightly overfitted on the shorter elongated member 42. The fine wirecoiled-coil heating element 50 is supported between these members 46,42. The coiled-coil barrel portion 50 is positioned intermediate theprojecting arm 46 and the shorter elongated member 42. In the specificembodiment 12 as shown in FIG. 3, a substantial portion of the coil end49 is embedded in the seal 38a. If desired, only the solid tungstenmember 42 need be embeded in the seal 38a. While the fine wire heatingelement has been specified as a coiledcoil member another embodiment isfor it to be comprised of a simple fine wire coil 60, as illustrated inembodiment 17 in FIG. 5.

The other electrode 14 consists essentially of tungsten and isoperatively positioned within the arc tube as shown in embodiment 12illustrated in FIG. 3, proximate the seal 38. This electrode comprisesan elongated tungsten support member 52 and a relatively heavy coiledtungsten member 54. This heavy coiled tungsten member 54 may be woundwith space between the turns for retaining a low-work-function material58 such as thorium oxide. A variety of other emissive materials that canbe used are well known in the art.

Lead-in conductors connect to the electrodes 14 and 16 through theconventional ribbon seals 30. As a specific example of the electrodeassembly, the solid tungsten member 40 has a diameter of 30 mils and thesolid tungsten member 42 has a diameter of mils. The overwound fine wiretungsten coil 50 is formed of wire having a diameter of 12 mils. Theprimary winding comprising the coil ends 48- and 49 has an interior coildiameter of 33.0 mils wound with 70 turns per inch. The coiled-coilbarrel portion 50 is formed of three secondary winding turns with aninterior barrel diameter of 60 mils, wound with 13 turns per inch. Afterthis electrode assembly is sealed into the arc tube, the projectingportions 56 of the hairpin-shaped assembly are cut away.

The electrode element 44 is constructed as an inner coil of 30 miltungsten, and an outer coil of 30 mil tungsten tightly wound atapproximately 28 turns per inch and substantially covering the innercoil. The electrode element 44 may be wound with space between the turnsfor retaining a low-work-function material 58. While this electrodeelement 44 comprises two concentrically wound coils of relatively heavytungsten wire more than just the two can be utilized. The coil isapproximately 5 mm. in length. The projecting arm portion 46 is also 30mil diameter and is affixed to or is part of this outer coil andprojects from the turn of the coil which extends furthest toward theother electrode. This type of relatively heavy coiled electrode isdescribed in detail by Rokosz, Pat. No. 3,170,081, dated Feb. 15, 1965.The electrode embodiment 14 has a similar coiled electrode element 54associated with it. This electrode element 54 is constructed the same aselectrode element 44 except there is no projecting arm portion from theouter coil of electrode element 54.

An alternative arc tube embodiment is shown in FIG. 4 wherein both endsof the arm tube 12a incorporate electrode-heating elements 16, asspecified hereinbefore. With such a construction, the lamp can bestarted at still lower voltage by heating at both ends of the arc tube,preliminary to applying the arc-initiating potential between theelectrodes. In other respects, this arc tube embodiment is similar tothe embodiment as shown in FIG. 3.

When the potential is initially applied to the lamp, the bimetal 32electrically connects directly to the embedded leg 49 of the heatingelement and the initial heating potential is applied directly across theelectrode. The coiled-coil heating element 50 rapidly heats to anincandescent status to heat the lamp. As soon as the bimetal is heated,it is electrically disconnected from the heating element 50 and as aconsequence, tl-ie potential is applied between the electrodes 14 and16. Since the projecting arm portion 46 projects farther into the arctube body portion than the heating element 50, the are quickly switchesfrom the heater element 50 to the more substantial extensive portion ofthe electrode. In electrode 16 this projecting arm portion 46 is anextension of the farthest projecting turn of the coiled electrodeelement 44. It is this electrode element 44 which acts as the primedischarge sustaining element after the heating element 50 initiates thedischarge.

When an additive lamp as shown in FIG. 1 is operated in conjunction witha conventional ballast and a starting circuit and incorporates theimproved electrode structures of the present invention as shown indetail in FIG. 3, the starting voltage averages less than volts. Ifheater elements are provided at both ends of the arc tube, as in theembodiment shown in FIG. 4, the average starting voltage will be evenlower. In either case, the starting voltage is substantially constantthroughout the life of the device.

It will be recognized that the objects of the invention have beenachieved by providing an improved arc-discharge device in which a finewire heating element is operatively positioned with respect to thepredetermined primary discharge sustaining surface. This determines thatthe discharge will quickly switch to the more stable electrode portionafter the discharge is initiated.

While the best embodiments of the invention have been illustrated anddescribed in detail, it is to be particularly understood that theinvention is not limited thereto or thereby.

I claim as my invention:

1. In combination with an arc-discharge device comprising an elongatedradiation-transmitting envelope having a seal at each end and containinga predetermined amount of discharge-sustaining filling, spacedelectrodes positioned within said envelope proximate each envelope seal,two spaced elongated tungsten support members of unequal lengthprojecting into said envelope from at least one of said seals, and atleast one of said electrodes comprising a fine wire coiled tungstenmember comprising end portions and a heating element portion supportedbetween said spaced tungsten support members, the improvement whichcomprises:

(a) a heavy coiled tungsten member tightly fitting over the inwardlyextending extremity of said longer 5 tungsten support member and formingan operating discharge-sustaining surface;

(b) a projecting arm portion affixed to the inwardly extending extremityof said heavy coiled tungsten member and generally directed toward butspaced from said shorter tungsten support member; and

(c) one end portion of said fine wire coiled member tightly overfittedon said projecting arm portion, and the other end portion of said finewire coiled member overfitted on said shorter tungsten support memher tosupport said fine wire coiled member therebetween in a position wheresaid projecting arm portion extends farther into the arc tube bodyportion than said heating element portion of said fine wire coiledmember.

2. The arc-discharge device as specified in claim 1, wherein alow-work-function substance is retained 'by said heavy coiled tungstenmember and said elongated support member.

3, The arc-discharge device as specified in claim 1, wherein saidelectrodes proximate each seal of said envelope include said fine wirecoiled member as a part thereof.

References Cited UNITED STATES PATENTS 2,315,286 3/1943 Hays et a1.313-33 X 2,524,455 10/ 1950 Macksoud 315-49 2,899,583 8/1959 Macksoud313-109 3,170,081 2/1965 Rokosz 313-217 X 3,356,884 12/1967 Decker et a1313-212 3,363,132 1/1968 Olson et a1. 313-198 JAMES W. LAWRENCE, PrimaryExaminer P. C. DEMEO, Assistant Examiner US. Cl. X.R.

